Oh my gosh... Keith is serious! In that case...
The lowest attenuating yeast I have ever used (at about 42%!) is 1099. So I'd use that over anything else.
Aim for a starting gravity around 1.070 and a final gravity around 1.030. Seems a good ballpark. This would give you just >5% ABV.
Go with the Gnome for most or all of the root beer flavor. I've made soft root beer with it a few times and it tastes good. Make it easy on yourself. If you want to add more personality, look into birch vs. wintergreen, see what's cheaper. Same maybe for licorice vs. anise. These would be additives on top of the Gnome. I'd still start with Gnome as a base flavoring to give you all the other in-between, and add the others to taste somehow or another (vodka tinctures maybe even??).
Hmm... what else would work here..... I dunno. Simplicity is probably best. Maybe 90+% pilsner malt with enough debittered black or Carafa III or whatever to darken. Or cheat and use Sinamar?! Or do both. I wouldn't use much if any caramel malts. If you use any molasses, keep it real slight. No more than like 1-2% of the fermentables.
Just thinking out loud. Can't say I'll ever try brewing this myself. Maybe this gives you a few ideas to bounce around anyway.